The attack involved two car bombs, a spokesman for Yemen's embassy in Washington said. Six attackers, including a suicide bomber wearing an explosive vest, were also killed in the attack, Mohammed al-Basha said.

There were at least four explosions -- including at least one car bomb -- and sniper fire, a senior State Department official said.

Yemeni officials said the first car contained people in police uniforms who exchanged fire with Yemeni security forces, the officials said.

The second car exploded after it passed an outermost gate to the Embassy but before it reached a second protective barrier, the officials said.

But al-Basha said there were two cars packed with explosives involved in the attack.

A U.S. official told CNN the attackers initially opened fire outside the embassy's security gate, then there was the main explosion followed by a secondary explosion.

At some point, snipers positioned across the street from the embassy opened fire on Yemeni first responders as they arrived on the scene, the official said.

Those killed include six Yemeni policemen and four civilians, he said, noting that the number of wounded is unclear. No U.S. Embassy employees were killed, the official added.

However, CNN affilliate WGRZ reported that Ahmed Elbaneh of Lackawanna, New York, said his sister, Susan, 18, and her new husband died in the attack.

Elbaneh said his sister left Lackawanna about a month ago for an arranged marriage and that she had been married for only 30 days. Elbaneh said family members in Yemen told him she was at the embassy translating for her new sister-in-law when the blast occurred.

Yemen believes al Qaeda is responsible for the attack, al-Basha said. State Department spokesman Sean McCormack said the the explosions "bear the hallmarks of an al Qaeda attack."

And Al-Basha said in a statement: "The government of Yemen considers terrorism as a crime against humanity. It strongly condemns this heinous attack. Whoever else who may have planned, plotted and supported the perpetrators of this terrorist attack will be apprehended and justice will prevail."

Media reports said Islamic Jihad in Yemen -- which is affiliated with al Qaeda -- has claimed responsibility for the attack, but CNN could not independently confirm those reports.

The U.S. State Department said it is sending a team of law enforcement and security experts to help in the investigation, spokesman Sean McCormack said.

"We heard the sounds of a heavy gun battle going on," he told CNN. "I looked out my window, and we saw the first explosion going off -- a massive fireball very close to the U.S. Embassy.

"The gun battle went on for a further 10 to 15 minutes, followed by two further loud explosions."

The first explosion happened about 9:15 a.m. Wednesday (0615 GMT/2.15 am ET) and was followed by several secondary blasts, said U.S. Embassy spokesman Ryan Gliha. Gliha describes the aftermath »

Gliha was at the embassy at the time of the attack and said he felt the compound shake.

"We were all ordered to assume what we call a duck-and-cover position which is a position where we guard ourselves and bodies from potential debris," Gliha told CNN.

"From that vantage point, I can't tell you much after that except we did feel several explosions after the main explosion that shook the ground."

Al-Basha called it a "despicable and heinous act" particularly because it took place during the Muslim holy month of Ramadan.

President Bush condemned the attack, and warned that it is "a reminder that we are at war with extremists who will murder innocent people to achieve their ideological objectives."

"One objective ... is they will try to cause the United States to lose our nerve," he said. "We want people to be able to live normal lives; we want mothers to be able to raise their sons and daughters."

A senior State Department official told reporters Secretary of State Condoleezza Rice, in a phone call with Yemeni president Ali Abdullah Saleh expressed regret for the loss of Yemeni life but also underscored the need to increase and improve cooperation against terrorism.

Witnesses told CNN they heard gunfire, and said they saw ambulances rushing from the scene.

The U.S. State Department has warned of violence that it attributes to Islamic extremists in Yemen. It has cited concern "about possible attacks by extremist individuals or groups against U.S. citizens, facilities, businesses and perceived interests."

The State Department ordered the departure of all non-emergency American staff from the Embassy, along with their family members, in April, after attacks against the Embassy and a residential compound. That order was lifted last month.

In March, three mortar rounds landed near the Embassy, injuring Yemeni students at a nearby school and Yemeni government security personnel, the State Department said.

The next month, an expatriate residential compound in the Hadda neighborhood was attacked by mortar fire. Suspected extremists fired two mortar rounds toward the Yemen Customs Authority and Italian Embassy in April, as well, but no one was hurt.